In wireless cellular communications virtually all communications are between a UE (User Equipment) and a base station, such as a U-ETRAN (Evolved Universal Terrestrial Radio Access Node) or eNB (evolved Node B). This ensures that communications are secure, interference in the network is managed, and account credentials are maintained.
D2D (Device-to-Device) communications are being developed to allow two remote devices to communicate directly with each other without going through a base station and the network infrastructure. While this is currently done in some cases, it requires a completely different wireless protocol, frequency bands and radios, such as NFC (Near Field Communication), Bluetooth or WiFi. Running D2D communications through a cellular or LTE (Long Term Evolution) radio allows the communications to be integrated seamlessly with other cellular interactions such as messaging and voice calls, etc. This has a possibility for reducing network traffic and reducing latency between users that are physically much closer to each other then they are to a base station. For users that are very close together there may also be a benefit of reducing radio transmission power which reduces interference for other users.
D2D may also be used for other types of services in which proximity or speed is important. In proximity-based services a UE is detected as being nearby and this triggers special services such as social applications, advertisements, local information about a venue or store, safety warnings, smart communication between vehicles, etc. Another service may be public safety where a user device can contact others nearby if the network infrastructure is damaged.
Network controlled D2D maintains a role for the cellular network infrastructure. Unlike NFC, Bluetooth, and WiFi, the network infrastructure controls and assists the efficient operation of D2D links. This is particularly valuable when D2D links coexist with cellular communications within the same shared cellular radio frequency bands. The cellular network infrastructure is already aware of other UEs and their locations and configurations. Accordingly, the network infrastructure is able to configure and establish D2D links more quickly than a UE by itself. The network infrastructure can also monitor links and handover a session to a conventional link through a base station if one of the UEs moves away from the other.
D2D discovery is intended to allow a UE to independently find nearby people, information, products, or services and then communicate directly with them with minimal interaction with the cellular network infrastructure. The use of LTE frequencies and protocols by multiple UEs without the control of a base station requires that the discovery process and sessions be integrated into the overall system of uplink and downlink messages.